Determination of Phosphate in water and waste water
Scope
This SOP is applicable for the determination of phosphate in water and waste water
Principle
In dilute orthophosphate solution, ammonium molybdate reacts under acid conditions to form a hetropoly acid, molybdo – phosphoric acid. In presence of vanadium, yellow vanadomolybdo – phosphoricacid is formed. The intensity of yellow Colour is proportional to phosphate concentration
Interference
Positive interference is caused by silica and arsenate only, if the sample is heated, Arsenate, fluoride, thorium, bismuth, sulphide, Thiosulphate, thiocyanate or excess molybdate cause negative interference. Ferrous iron results in blue colour and does not affect the results, if the concentration is less than 100 mg/l.
Method (method A)
Vanadomolybdo-Phosphoric acid method (By using Spectrophotometer)
(Minimum detectable concentration 200 µg of phosphorus per lit in 1 cm spectrophotometer cell)
Reagents
- Phenolphthalein solution
- HCL 1:1
- Activated Carbon
- Vanadate Molybdate Reagent
Dissolve 25 g of ammonium molybdate in 300 ml of distilled water (Solution A). Dissolve 1.25 g of ammonium metavandate by heating to boiling in 300 ml of distilled water. Cool and add 330 ml of conc. HCL (Solution B) Cool to room temperature. Pour solution A to solution B, Mix and dilute to 1L.
Standard Phosphate solution
Dissolve 0.2195 gm of anhydrous potassium dihydrogen phosphate (KH2PO4) in distilled water and dilute to 1000ml, 1 ml = 50 µg orthophosphate phosphorus.
Calibration
Prepare a series of phosphate standards by diluting 0 ml, 2 ml, 5 ml, 10 ml, 20 ml, 30 ml, 40 ml, 50 ml of phosphate stock solution (1 ml = 0.05 mg PO4) to 100 ml with water in a volumetric flask. And proceed as per procedure (7.0)
Procedure
1. If the sample pH is greater than 10, add 0.05 ml of phenolphthalein indicator to 50.0 ml of sample and discharge the red Colour with 1:1 hydrochloric acid before diluting to 100 ml.
2. Remove excessive Colour in sample by shaking about 50 ml with 200 mg of activated carbon in
an Erlenmeyer flask for 5 minutes and filter to remove carbon.
3. Place 35 ml or less of sample containing 0.05 to 1.0 mg phosphorus in a 50 ml volumetric flask.
4. Add 10 ml vanadate-molybdate reagent and dilute to the mark with distilled water.
5. Prepare a blank in which 35 ml of distilled water is substituted for the sample. After 10 minutes or more, measure absorbance of sample versus blank at a wavelength of 470 nm. The colour is- stable for days and is not affected by variation in room temperature.
Calculation
Phosphorus (P) mg/l = m/v x 1000
Where,
m= mg of phosphorus (in 50 ml of final volume)
v = volume in ml of sample
Method (Method B)
Stannous chloride method (By using Spectrophotometer)
(The minimum detectable concentration by this method is about 3 µg/l of phosphorus.)
Principle
The molybdo-phosphoric acid formed is reduced to an intensely coloured complex molybdenum blue by stannous chloride. This method is significantly sensitive and the reliability of the method increases at concentrations below 0’1 mg/l of phosphorus with minimum interference.
Interference
Silica and arsenic interfere positively, if the sample is heated. Arsenate, fluoride, sulphide, Thiosulphate, thiocyanate or excess molybdate cause negative interference. Ferrous iron which causes blue colour does not affect the result if the concentration is below 100 mg/l. If nitric acid is used in the test, chloride interferes at 75 mg/l.
Reagents
Phenolphthalein indicator solution
Strong acid solution
Add slowly 300 ml of sulphuric acid to about 600 ml of DM water, cool and add 4 ml of nitric acid and dilute to 1 litre.
Ammonium molybdate reagent
Dissolve 25 g of ammonium molybdate in 175 ml of distilled water. Continuously add 280 ml of concentrated sulphuric acid to 400 ml of distilled water in a separate beaker cool and add the molybdate solution to this acid solution and dilute to 1 litre.
Standard Phosphate solution
Dissolve 0.2195 gm of anhydrous potassium dihydrogen phosphate (KH2PO4) in distilled water and dilute to 1000ml, 1 ml = 50 µg orthophosphate phosphorus.
Stannous chloride solution
Dissolve 2.5 g of a fresh stannous chloride in 100 ml of glycerol. Heat in a water bath and stir with a glass rod to hasten dissolution. This reagent is stable and requires neither preservatives nor special storage.
Activated carbon
Calibration
Prepare a series of phosphate standards by diluting 0 ml, 2 ml, 5 ml, 10 ml, 20 ml, 30 ml, 40 ml, 50 ml of phosphate stock solution (1 ml = 0.05 mg PO4) to 100 ml with water in a volumetric flask. And proceed as per procedure (5.0)
Procedure
Note: If coloured, decolourize the sample by shaking about 200 ml sample with 256 mg of activated carbon in an Erlenmeyer flask for 5 minutes. Filter the solution through filter paper (Whatman No. 42) to remove carbon.
Take 100 ml clear and colourless sample add 1 drop phenolphthalein indicator, if pink color develop, discharge color by strong acid solution. the acid requirement exceeds 5 drops, take a smaller sample and dilute to 100 ml with distilled water, after first discharging the pink colour with acid.
Add 4.0 ml molybdate reagent and 0.5 ml of stannous chloride reagent.
wait for 10 min and before 12 min measure the color spectrophotometrically at 690 nm and compare with calibration curve using DM water blank
Calculation
Phosphorus mg/l = mg of phosphorus corresponding to control std. X 1000
Volume in ml of sample
Safety guidelines
Proper safety & health precaution are taken while doing the analysis by using safety goggles, apron, safety shoes & using fume chamber for fume generating chemicals
Reference
IS: 3025 (Part-31) 1988 RA 2019